Nodes in low-power and lossy networks (LLNs) are constrained by processing power, memory, power consumption, lifetime, rate of activity, and physical size. The communication links between the nodes can be characterized by high loss rate, low data rate, instability, low transmission power, and short transmission range. There can be from a few dozen up to millions of nodes within a practical LLN. Examples of LLN include a smart meter network, and a wireless sensor network for building monitoring.
In contrast with mobile ad-hoc networks, the LLN can have constrained traffic patterns, although multipoint-to-point traffic, e.g., from nodes to one or more data sinks or concentrators, dominates. The point-to-multipoint and point-to-point traffic rare. The data sink is usually a control point and collects data from nodes in the LLN. The LLN can have multiple data sinks for reliability and efficiency. Data sinks can work cooperatively or independently and can be connected to a data center via network interfaces.
Large scale LLN deployment and a high volume of packet delivery can result in packet losses due to the above constraints. Retransmission can increase network traffic. To increase reliability and reduce overhead, nodes in the LLN with multiple data sink nodes can select optimal routes. However, most conventional routing methods for LLNs are not designed for multi-sink routing.
Several conventional multi-sink routing methods are known. However, those methods are not optimal for LLNs and unsynchronized. For example, the Internet engineering task force (IETF) has developed the IPv6 Routing Protocol for LLNs (RPL), which uses a directed acyclic graph (DAG) to discover routes. Even though RPL is a multi-sink routing protocol, routes are discovered based on building a Destination Oriented DAG (DODAG), which has a single sink node called the DODAG root. After the routes are discovered, a node transmits packets to the DODAG root. RPL does not specify how a node discovers and maintains routes to multiple sinks and how a node switches between multiple sinks. Also, the DODAG is unsynchronized.
The synchronization is required in some LLNs. For example, the synchronization can be used by smart meter network for multi-purpose operations, including metering data collection, demand and response management, control of smart meters and devices connected to smart meter network. As a result, RPL may not be appropriate for multi-sink multi-purpose routing in some LLNs.
U.S. 2012/0236855 describes a method for controlling multi-sink/multi-path routing for a sensor network. That method is designed for safety-critical system. To satisfy the reliability requirement, that routing method transmits multiple copies of the same data to multiple data sinks using multiple paths. The overhead for that method is excessive for large scale LLNs. Also, multi-sink multi-path multiple transmissions can decrease the reliability of the LLNs.